The present invention relates to an antenna for a portable radio unit and, more particularly, to an antenna for a portable radio unit obtained by combining a helical antenna element and an extendible, retractable whip antenna element.
Various types of antennas are used as portable radio antennas. A whip antenna is desirable in the communication mode in terms of obtaining a maximum gain by decreasing degradation in gain caused by the human body, and a small antenna, e.g., a helical antenna, is desirable in the wait mode as it can be conveniently carried around.
Accordingly, an antenna obtained by combining a helical antenna element and an extendible whip antenna element is conventionally known. In the communication mode, the whip antenna element is extended and operated. In the wait mode, the whip antenna element is retracted and the helical antenna element is operated.
According to an example of the arrangement of the conventional antenna of this type, for example, a helical antenna element is formed on the distal end of a whip antenna element. When the whip antenna element is extended, power is supplied to the whip antenna element. When the whip antenna element is retracted, power is supplied to the helical antenna element.
The diameter, pitch, and length of the helical antenna element must be determined in accordance with the transmitted and received carrier frequency, so that desirable relative bandwidth and resonant frequency can be obtained. Also, in order to decrease degradation in gain caused by the human body during communication, the length of the whip antenna element must be set to about .lambda./2. Therefore, the input impedances of the two antenna elements are usually largely different from each other, and it is thus difficult to realize an optimum matching state when the whip antenna element is extended and retracted.
As described above, in the conventional antenna obtained by combining the helical antenna element and the whip antenna element, the following method must be inevitably used. The antenna is used while allowing mismatching. Alternatively, the length of the whip antenna and the size of the helical antenna are changed from the above-mentioned determined values so as to facilitate matching.